Field
The present disclosure relates generally to energy storage devices, and more particularly to lithium-ion battery technology and the like.
Background
Owing in part to their relatively high energy densities, light weight, and potential for long lifetimes, lithium-ion (Li-ion) batteries are used extensively in consumer electronics. In many applications, Li-ion batteries have essentially replaced nickel-cadmium and nickel-metal-hydride batteries. Despite their increasing commercial prevalence, further development of Li-ion batteries is needed, particularly for potential applications in low- or zero-emission hybrid-electrical or fully-electrical vehicles, consumer electronics, energy-efficient cargo ships and locomotives, aerospace, and power grids. Such high-power applications will require electrodes with higher specific capacities than those used in currently-existing Li-ion batteries.
Currently, carbon-based materials (e.g., graphite) are employed as the predominant anode material in Li-ion batteries. Carbon (C), in the form of graphite, has a maximum or theoretical specific capacity of about 372 milli-Ampere hours per gram (mAh/g), but suffers from significant irreversible capacity losses during the first formation cycling. Notably, during the first charge cycle, the battery electrolyte decomposes on the graphite surface and a significant number (typically in the range of about 5%-15%) of lithium ions present in the cathode during the initial charge (or intercalated into the graphite during charging) become buried in the decomposed layer of electrolyte and cannot be extracted therefrom upon discharge of the battery. The higher specific surface area of other low voltage (e.g., about 0 V-1.2 V vs. Li/Li+) anode materials generally enhances the degree of electrolyte decomposition and, hence, the magnitude of the irreversible capacity losses.
Capacity storage in a Li-ion battery anode may be improved to a degree by substituting graphene for graphite to increase the number of lithiation sites (i.e., sites for the storage of lithium ions during charging). However, graphene-based anodes are susceptible to other problems, including still very high irreversible capacity losses upon initial cycling (due to their high specific surface area, on which the electrolyte decomposes), correspondingly low Coulombic Efficiency (CE) during both the first and also subsequent cycling, and limited overall stability caused by the separation of graphene layers. While composite electrodes employing graphene nanosheets, carbon nanotubes, and/or fullerenes have been shown to increase capacity, they have failed to provide the level of stability required for widespread adoption in industry. Graphene therefore remains unsuitable as a replacement to current graphite-based anode technology.
A variety of higher capacity materials have been investigated to overcome the drawbacks of carbon-based materials. Silicon-based materials, for example, have received great attention as anode candidates because they exhibit specific capacities that are an order of magnitude greater than that of conventional graphite. Silicon has the highest theoretical specific capacity among metals, topping out at about 4200 mAh/g. Unfortunately, silicon and similar materials suffer from their own significant challenges.
One limitation of silicon is its relatively low electrical conductivity. Another limitation is the relatively low diffusion coefficient of lithium in silicon, leading to a low conductivity of lithium ions permeating silicon. The primary challenge of silicon-based anode materials, however, is the volume expansion and contraction that occurs as a result of lithium ion alloying or dealloying, respectively, during charge cycling of the battery. In some cases, a silicon-based anode can exhibit an increase and subsequent decrease in volume of up to about 400%. The high level of strain experienced by the anode material can cause irreversible mechanical damage to the anode. Ultimately, this can lead to a loss of contact between the anode and underlying current collector.
To mitigate such detrimental effects, various composite electrodes formed from higher capacity materials and different carbon arrangements have been explored, ranging from so-called three-dimensional (3-D) micron-sized particle structures, to one-dimensional (1-D) nanotube structures, to zero-dimensional (0-D) nanoparticle structures. The different structures have been fabricated by a corresponding variety of techniques, including physical mixing, decomposition of C- and Si-containing precursors, and other solution-based methods. However, each of these conventional structures has failed to provide the level of performance required for widespread adoption. Many suffer from limited porosity available for volume changes in the active material, non-uniform material properties at the nanoscale, a high surface area that leads to very large irreversible capacity losses and low CE upon initial cycling as well as during subsequent cycling, and other problems.
Many high capacity cathode materials for Li-ion batteries also suffer from low electrical conductivity, a low diffusion coefficient of lithium, high volume changes during battery operation, or a combination of such shortcomings
Accordingly, despite the advancements made in electrode materials, high capacity Li-ion batteries remain somewhat limited in their applications and there remains a need for improved electrodes for use in Li-ion batteries. Improved anodes and cathodes and, ultimately, the improved Li-ion batteries, could open up new applications and advance the adoption of so-called high-power devices such as those discussed above.